The present invention relates to an air/fuel ratio control system for an internal combustion engine. More particularly, the invention provides an air/fuel ratio control system for an internal combustion engine, which enables the internal combustion engine to be started at an optimum air/fuel ratio by controlling the amount of fuel injected at each fuel injection when the engine is in the process of being started.
It is known to control air/fuel ratio according to various engine operating parameters so as to run the engine under an optimum condition. The same control of the fuel supply amount for each injection carried out during normal running of the vehicle is also carried out when the engine is in the process of being started. The injection time of a fuel injector is suitably set to provide fuel to the engine at a rate according to the starting characteristics of the engine. However, there is a problem in using the "normal" air/fuel ratio controlling techniques during engine starting.
Systems which determine the amount of a fuel to be supplied to the engine in proportion to the injection time of the fuel injector do not operate properly when the engine is being started "hot" (i.e. the engine was recently operated and turned off). Under a "hot" start condition the actual amount of fuel supplied to the engine is reduced from the amount it should be because some of the fuel is vaporized by the heat of the engine. The amount of fuel is reduced by a rate at which fuel vapor is being generated. For a given injection time the air/fuel ratio becomes leaner than desired. This will occur when the engine is heated to a high temperature after a high-load run over a long time. This phenomenon is particularly likely to occur when the engine is started "hot". The engine may fail to be supplied with a sufficient amount of fuel thereby making the engine difficult, if not impossible to start.
As disclosed in Japanese Pre-Examination Patent Publication No. 59-134335, there has been proposed a system for increasing the injection amount of the fuel (over and above the amount that would otherwise have been injected) at the start of the engine when the engine is in its state of high cooling water temperature, by detecting not the fuel temperature but the cooling water temperature.
However, even the proposed system does not completely solve the problem. As seen in the Figure 9 graph, fuel temperature in the injector (indicated by reference numeral 902) which is plotted against time lapse from an engine stop, is not identical to cooling water temperature (indicated by reference numeral 901) plotted against time lapse from engine stop. It is, therefore, not always effective to simply increase the fuel supply rate when the cooling water temperature exceeds some predetermined judgment level (indicated by reference numeral 903), as in the prior art system.
With continued reference to FIG. 9, consider the case in which five minutes have elapsed after engine stop. Although fuel temperature in the injector is not so high at 60.degree. C., cooling water temperature exceeds judgment level 903 so that the fuel supply amount is increased. This causes a problem in that the fuel supply amount is increased even for a small amount of fuel vapor generated so that the air/fuel mixture is abnormally enriched, which deteriorates fuel consumption rate and exhaust emissions. Consider another case in which thirty or more minutes have elapsed after engine stop. Although the cooling water temperature is below judgment level 903 so that there will be no fuel supply increase, vapor lock may occur because the fuel temperature is still high. The proposed system would compensate by leaning the air/fuel ratio which would make it more difficult to start the engine.